CVD is a chemical process used to produce high-purity, high-performance solid materials. The process is often used in the semiconductor and photovoltaic industries to produce high quality silicon materials. In a conventional CVD process, a filament or rod structure is exposed to one or more volatile precursors that react and/or decompose on the filament surface to produce the desired deposit. Frequently, volatile byproducts are also produced, which can be removed by gas flow through the reaction chamber of the CVD reactor.
One method used to produce solid materials such as polysilicon in a CVD reactor is the Siemens method, in which polysilicon is deposited on thin silicon filaments. Because the filaments are fabricated from high-purity silicon, the electrical resistance of the filaments during a reactor startup phase is extremely high. Unless the filaments are doped with an electrically active element, the high electrical resistance makes it difficult to heat the filaments using electrical current during the startup phase.
To accelerate the heating process during startup, a high voltage, on the order of thousands of volts, may be applied to the filaments. This causes a small electrical current to flow through the filaments, which generates heat in the filaments. As the filaments heat up, the electrical resistance of the filaments is reduced, which permits yet higher current flow and additional heat to be generated by the filaments. When the filaments reach the desired temperature, typically greater than 800° C., the voltage may be reduced so that further temperature increases do not occur.
In some instances, due to vibration, loose connections, forces associated with fluid flow inside the reactor, and/or other causes (e.g., the weight of deposited materials), a filament in a CVD reactor may tilt or tip over and come into contact with the reactor wall or another filament in the reactor. Such contact generally causes a ground fault in the reactor, resulting in termination of the CVD process, and costly downtime. Although there are cost and production advantages to using tall and thin filaments, such filaments are more likely to tip over, often due to breakage of the filament (e.g., near the base plate, at a chuck-to-filament connection). Further, as polysilicon is deposited on the filaments, the added weight of the polysilicon puts stress on the filaments and increases the likelihood that the filaments may break and/or tip over.
There is a need for methods, systems, and apparatus for stabilizing filaments in a CVD reactor to prevent ground faults and unnecessary stoppages of CVD processes.